Soil information is publicly available in the United States as air photos overlaid with closed boundaries or polygons each enclosing a geographic region or “soil map unit” labeled with a number corresponding to a specific subsurface material characteristic reference profile identified in a United States Department of Agriculture—Natural Resource Conservation Service (USDA-NRCS) Soil Survey as a soil series description. Such soil surveys are presently available for over ninety percent of the geographic area of the United States, and other types of soil characterization maps are also available, both for the United States and other geographic regions. In any given field there may be several polygons, each of which bounds a numbered soil map unit. On average, there may be about 5.5 polygons per any 100 acre area. As the number of the soil map unit corresponds to a particular characteristic soil type and profile, many soil map units in a particular area may have identical labels. A table is used in the survey to link each number to a soil series name, and for each soil series name, the Soil Survey contains a text description of a representative soil profile. For example, the listing for the “Drummer Series” corresponding to soil map unit “152” is as follows:
The Drummer series consists of deep, nearly level, poorly drained soils. These soils are in drainageways and in broad, flat areas on uplands. They formed in silty material and in the underlying stratified loamy material.
In a representative profile the surface layer is black and very dark gray silty clay loam about 18 inches thick. The subsoil, about 32 inches thick, is olive-gray and gray to light-gray silty clay loam mottled with strong brown. The underlying material is gray and strong-brown sandy clay loam and silt loam.
These soils are moderately permeable and have high available water capacity. Surface drainage and internal drainage are very slow, and when wet, the soils have low stability. The soils are subject to frequent water-logging and flooding or ponding during periods of run-off. Seasonally the water table is at a depth of less than 2 feet.
These soils are well suited to row crops if they are adequately tile drained. Frequent waterlogging and potential for flooding or ponding are sever limitations for most other uses.
Representative profile of Drummer silty clay loam, 480 feet south and 75 feet west of the N.E. corner of SE1/4 sec. 24, T. 39 N., R. 3 E.
A11—0 to 11 inches, black (10YR 2/1) silty clay loam; moderate, fine and very fine, granular structure; friable; neutral; gradual, smooth boundary.
A12—11 to 18 inches, black (10YR 2/1) and very dark gray (10YR 3/1) silty clay loam; weak, fine, subangular blocky structure parting to moderate, fine, granular; friable; neutral; gradual, smooth boundary.
B21g—18 to 29 inches, olive-gray (5Y 5/2) silty clay loam; few, fine, faint, strong-brown (7.5YR 5/6)mottles; moderate, fine, prismatic structure parting to moderate, fine, subangular blocky; firm; neutral; gradual, smooth boundary.
B22g—29 to 50 inches, gray to light-gray (5Y 6/1) and strong-brown (7.5YR 5/6) silty clay loam; moderate, medium, prismatic structure; firm; neutral; abrupt, smooth boundary.
HC1g—50 to 60 inches, strong-brown (7.5YR 5/8) and gray to light-gray (5Y 6/1) sandy clay loam; massive; friable; mildly alkaline; abrupt, smooth boundary.
HC2g—60 to 78 inches, gray to light-gray (5Y 6/1) and some pinkish-gray (5YR 6/2), brown to dark-brown (7.5YR 4/4) and gray to light-gray (7.5YR 6/0) silt loam; massive; friable; moderately alkaline.
The A horizon ranges from 8 to 20 inches in thickness. The B horizon ranges from 15 to 40 inches in thickness and from silty clay loam to clay loam in texture in the lower part. It is neutral to mildly alkaline. The C horizon ranges from loam to silt loam to sandy clay loam.
Drummer soils are associated with Peotone and Flanagan soils. They have a thinner A horizon than Peotone soils and are more poorly drained than Flanagan soils.
This text-based description of the soil series includes textual soil log information from a representative profile located somewhere in the United States. This soil type and profile information is useful to estimate the soil characteristics of any soil map unit of that soil series (in this case, the Drummer Series number 152) located anywhere in the United States or elsewhere.
These representative profile descriptions typically range from the surface down to about 60 to 80 inches and contain descriptions of horizons or soil layers. Information on horizon depth and thickness and soil attributes is given for each typical soil horizon. This data is at best a representation of what that particular type of soil might look like on average in a typical situation. Thus, at any given location there will be differences between this characteristic information and actual boundary layer depth and thickness, and to some extent the soil content and type within each boundary or horizon. In order to better classify soils for precision management applications, it is sometimes necessary to characterize the actual depth, thickness and content of a particular soil map unit within a field, and all other soil map units contained within that field.
The USDA-NRCS polygons are available currently in digital format for about sixty percent of the United States. In these cases, the soil map unit boundaries and their landscape position can be downloaded from a database accessible through the Internet, or from a CD-Rom or other data storage device. In some cases, the air photo is also available in digital format. However, the text descriptions of the soil series are currently only available in text format that can be read and viewed by an individual, but not readily compared digitally to sample or sensor data. Also, as the description associated with a particular soil series is almost always from a representative soil log taken at a location far from the actual field polygon that is being mapped, various inaccuracies can develop.
The USDA-NRCS is responsible for mapping soils in the United States at a scale referred to as a Second Order Survey. This is typically in a scale range from 1:24,000 to 1:60,000. It is generally understood that such rough scales are not particularly useful for high precision applications, such as agriculture, wetland mapping, construction, watershed, golf course design maintenance, archaeological mapping, environmental site assessments, and such. Methods are needed for more accurately characterizing subsurface conditions in geographic regions previously associated as a whole with a specific subsurface material characteristic reference profile in a USDA-NRCS survey or the like, to make such database information more useful for such precision applications.
The depth and thickness of soil horizons and their properties can vary immensely within a landscape, and even within a given field. If a critical soil property, such as nutrient and water holding capacity or carbon content, is to be assessed within a given field or area, then it is critical that the vertical and horizontal distribution of such properties be determined accurately. When a soil core is collected, the number of sections analyzed in the sample limits the vertical resolution of the soil property assessment at that location. This is due primarily to the high cost and time expenditure associated with soil sample collection, preparation, analysis, and recording procedures. Typically, only a few locations across a landscape are chosen for core sampling, and only a few sample sections are removed from each core for analysis. This limited vertical soil information results in errors when attempting to model the spatial distribution and volume of soil properties across a landscape.